GAZA BORDER (Reuters) - Sometimes photographers know they have something special in the fraction of a second between seeing it happen and capturing it through their lens.

Mid-afternoon on a scorching hot Friday near the Gaza-Israel border east of Gaza City was one such moment for Reuters photographer Mohammed Salem, with the smell of burning rubber in his nostrils and the constant fear of bullets and tear gas in his mind.

Seventeen years of experience as a photographer in the Gaza Strip meant that Salem knew that Friday was likely to be a busier day than others at the protest camps that have sprung up along the Gaza-Israel border in the last week.

Since March 30, thousands of Palestinian demonstrators have been rolling burning tires and throwing stones, some using slingshots, in daily clashes with Israeli soldiers on the other side of the border fence.

It is a potentially deadly assignment. In the week since the protests began on March 30, as of Friday 27 Palestinians have been shot dead, and journalists have been injured. So Salem went to the scene early, before the protesters arrived, wearing his helmet and flak jacket clearly marked "Press" and carrying a gas mask.

It was between 2:00 p.m. and 3:00 p.m. as the protests intensified that the smoke suddenly cleared and the moment presented itself.

"I was 300 meters away from the fence. Those youths were calling to other young men, urging them to come forward in order to cross the border fence," Salem recalls.

"The whole place was covered in heavy smoke rising up from the many tires that had been burnt. They came closer to each other, in excitement. In the background Israeli forces fired heavy tear gas, and tried to extinguish the burnt tires.

"I knew it was a good picture, and a strong one, the moment I saw the scene."

As soon as he got the picture, the youths disappeared through the thick black smoke toward the border, and the Israeli sharpshooters.

It was far too dangerous for him to go with them, he calculated, and anyway the very same black smoke which had made the shot so dramatic had now closed in completely, making photographs impossible. Crucially, the smoke left him with no way of knowing what was going on around him. So it was time to leave.

He knew what he had, and went straight to the Reuters office in Gaza City to file, so he has no idea what happened to the youths in the image.

"I felt the picture was full of anger and excitement," Salem concluded.